The co-pilot of Solar Impulse 2 says the aircraft is now ready to head off from its unplanned stopover in Japan but must wait out unfavourable weather, perhaps for as long as two months.

Andre Borschberg, co-pilot of the solar-powered plane that began its round-the-world flight attempt in Abu Dhabi, said it could take weeks for a weather front stretching from Alaska to Taiwan to clear enough for him to leave Nagoya, in central Japan.

"The goal is not to take risks but to get there safely,” the Swiss national said in Tokyo.

"The front, physically, is like a wall. It's too cloudy. It's too rainy. It's too bumpy,” he said. "What we need is to find a weak spot in this ‘wall' so we can fly over.”

Mr Borschberg diverted to Nagoya because of weather worries while travelling from Nanjing in China to Hawaii, at 8,175 kilometres, the longest leg of a journey that began in the UAE on March 9.

He said the plane, which was slightly damaged by a cover shaken by the wind while on the ground in Nagoya, is fully repaired and ready to go.

The flight to Hawaii will take five or six days. The aeroplane carries no fuel, so project engineers use simulations to decide if it is safe to fly.

Mr Borschberg and the project's leader, Bertrand Piccard, are taking turns flying the plane solo. Since the plane is meant to travel during the summer, there is a limit to how long they can wait to make the flight this year.

"I hope we will find a time in the next two weeks,” Mr Borschberg said. "What makes this flight very difficult is that we are at the very limit of what the weather forecasting can do today and we don't want to take too much of a risk.”

The plane is powered by more than 17,000 solar cells on its wings that recharge its batteries, enabling it to fly.

The goal of the project is less about solar-powered air travel, which would not be commercially practical given weather and weight constraints of the Solar Impulse, but about spreading a message about clean technologies.

The Solar Impulse's extra-wide wings, brightly lit as it flew into Nagoya, caused some Japanese to call authorities to report a UFO, or unidentified flying object.

"The technologies that we use to illuminate the airplane that makes everyone think it's a UFO ... can be used anywhere on the ground,” Mr Borschberg said. "What we try to say with this project is if we would use these technologies in homes, cars, appliances, we could drastically reduce energy consumption on the ground.”